[论文解读] Simultaneous X-ray and optical observations of thermonuclear bursts in the LMXB EXO 0748-676

Biswajit Paul, M. Archana+1 more|arXiv (Cornell University)|Mar 8, 2012

Astrophysical Phenomena and Observations被引用 6

一句话总结

本研究利用XMM-Newton数据，首次在食变低质量X射线双星EXO 0748-676中实现了X射线与光学热核爆发的同时大样本探测。研究发现，X射线与光学爆发起始之间存在平均3.25秒的延迟，且该延迟与轨道相位无显著依赖关系，这挑战了光学爆发源于伴星表面再处理的假设，表明吸积盘更可能是主要的再处理区域。

ABSTRACT

We report the detection of a large number of optical bursts in the Low Mass X-ray Binary (LMXB) EXO 0748-676 simultaneous with the thermonuclear X-ray bursts. The X-ray and the optical bursts are detected in a long observation of this source with the XMM-Newton observatory. This has increased the number of thermonuclear X-ray bursts in the LMXBs with simultaneous optical detection by several factors. The optical bursts are found to have a linear rise followed by a slow, somewhat exponential decay. Most of the optical bursts have longer rise and decay timescale compared to the corresponding X-ray bursts. We have determined the X-ray and optical excess photon counts in the bursts that allow us to look at the optical to X-ray burst fluence ratio for each burst and the ratio as a function of the X-ray burst intensity and as a function of the orbital phase. The delay between the onset of the X-ray bursts and the onset of the optical bursts have also been measured and is found to have an average value of 3.25 seconds. We do not find any convincing evidence of orbital phase dependence of the following parameters: X-ray to optical delay, rise time of the optical bursts, and optical to X-ray burst intensity ratio as would be expected if the optical bursts were produced by reprocessing from the surface of the secondary star that is facing the compact star. On the other hand, if the optical bursts are produced by reprocessing of the X-rays in the accretion disk, the onset of the bursts is not expected to have a sharp, linear shape as is observed in a few of the bursts in EXO 0748-676. We emphasise the fact that simultaneous optical observations of the X-ray bursts in multiple wavelength bands will enable further detailed investigations of the reprocessing phenomena, including any non-linear effect of the X-ray irradiation.

研究动机与目标

• 通过分析光学爆发与X射线爆发的时序和光谱特性，探究EXO 0748-676中光学爆发的起源。
• 检验光学爆发是否如盘面照射模型所预测的那样，源于伴星被照射面的X射线光子再处理。
• 测定光学与X射线爆发的能量通量比及其与X射线爆发强度和轨道相位的关系。
• 通过分析X射线与光学爆发起始之间的几何延迟，推断再处理区域的位置与结构。
• 评估多波段光学/紫外观测在探测低质量X射线双星中非线性再处理效应的可行性。

提出的方法

• 分析档案中的XMM-Newton观测数据，结合EPIC-pn的X射线数据与光学监视仪的光学/紫外数据。
• 通过光 light curve 分析识别X射线与光学爆发，重点关注爆发的上升与衰减阶段。
• 利用互相关技术测量X射线爆发起始与光学爆发起始之间的时间延迟。
• 计算单个爆发的光学与X射线爆发能量通量比（光学转换因子），并分析其与X射线爆发强度和轨道相位的关系。
• 评估爆发延迟、上升时间与光学转换因子的轨道相位依赖性，以区分伴星表面与吸积盘再处理机制。
• 利用光行时估算（约3秒）约束再处理区域的几何结构。

实验结果

研究问题

• RQ1 光学爆发起始是否相对于X射线爆发起始存在延迟？若有，延迟时长为多少？
• RQ2 若再处理发生在伴星被照射面，则X射线到光学爆发的延迟是否随轨道相位变化？
• RQ3 光学与X射线爆发的能量通量比是否恒定？其是否随X射线爆发强度或轨道相位而变化？
• RQ4 光学爆发的上升与衰减 timescale 是否与X射线爆发显著不同？
• RQ5 观测到的光学爆发线性上升与指数衰减特征是否可由吸积盘中的再处理过程解释？若不能，是否与基于吸积盘的模型相矛盾？

主要发现

• 共检测到23次X射线与光学同时爆发，显著增加了具有多波段探测的低质量X射线双星样本数量。
• X射线爆发起始与光学爆发起始之间的平均延迟为3.25秒，与双星系统中光行时延迟一致。
• 在X射线到光学的延迟、光学爆发上升时间以及光学与X射线能量通量比方面，均未发现与轨道相位的显著依赖关系，这不利于伴星表面再处理的解释。
• 光学爆发呈现线性上升与缓慢的指数衰减，其时间尺度长于对应的X射线爆发。
• 不同爆发之间的光学与X射线能量通量比变化幅度可达五倍，平均值约为300，但因光学光谱特性未知，存在不确定性。
• 观测到的爆发形态——特别是陡峭的线性上升——对基于吸积盘的再处理模型构成挑战，提示存在尚未被考虑的复杂或非线性再处理效应。